This invention relates to a random access system and, more particularly, to a random access system for use in a recording and/or reproducing apparatus wherein predetermined addresses corresponding to selectable portions of a record medium can be recorded on and reproduced from the record medium, the address signals exhibiting a particular format which minimizes false operation.
In many recording/playback devices, such as tape recording apparatus, it is desirable to access random portions of the record medium, such as the record tape, so as to reproduce the information which had been recorded previously in such portions without requiring the reproduction of all of the information signals that are not in interest. That is, if the tape is moved from a first position therealong to a desired position, it often is preferred not to play back the signal information which had been recorded between such positions.
In a basic type of random access system, a predetermined pulse signal, such as a marker pulse, had been recorded on the record medium in advance of, or at the beginning portion of the useful signal information. Thus, if the signal information is recorded in successive, variable-length blocks, such marker pulses are recorded in between successive blocks. Then, if it is desired to access a particular block so as to reproduce only the signal information recorded in that block, the marker pulses are counted as the record medium is advanced accordingly. Then, when the proper number of such pulses had been counted, the desired block is in position for a playback operation. Unfortunately, with this type of basic random access system, all block-accessing must be referenced to the initial, or starting block. If initial operation of the record medium commences at some arbitrary, intermediate position, the marker pulse count may not correspond to the desired block location. For example, if the record medium is a magnetic tape, and if this tape is loaded onto suitable playback apparatus such that the first block which is positioned for a playback operation is, for example, block #6, then the counting of two marker pulses in, for example, the forward direction, will position the tape at block #8, rather than at block #2 which may have been the desired block.
One proposal to overcome this drawback of a basic random access system is to record, in advance of each block of information, a number of pulses corresponding to the block number. Thus, these pulses correspond to an address to identify the immediately following block of information. For example, one pulse will be recorded at the beginning portion of block #1, two pulses will be recorded at the beginning portion of block #2, three pulses will be recorded at the beginning portion of block #3, and so on. Then, during a playback operation, a desired block number can be accessed merely by counting the number of block-identifying pulses until the desired count is obtained. However, an attendant disadvantage of this type of random access system is that a spurious pulse may be erroneously interpreted as a block-identifying pulse and will be counted. For example, if the record medium comprises a record tape, such a spurious pulse might be located in the course of an information signal. Hence, when the record tape is scanned for the purpose of reading, or reproducing such block-identifying pulses, this spurious pulse will be detected and may erroneously alter the pulse-count. That is, there is no simple technique which can be used to distinguish between actual block-identifying pulses and spurious pulses.